When rotary drilling an underground wellbore from the surface, a drilling fluid in the wellbore is typically used to prevent wellbore wall caving and prevent the intrusion of formation fluid, such as unwanted oil, gas, and water. Another important function of the drilling fluid (typically a "drilling mud" mixture) is to entrain drilled cuttings and circulate them to the surface and out of the borehole. The drilling fluid typically also cools and lubricates the moving drill string components and strikes the drilling face of the underground formation with an impact force that may further assist in drilling.
Although density, viscosity, and surface pressures on the drilling fluid are controlled, the density of the drilling fluid is the most important to control in order to provide a hydrostatic pressure in excess of formation pore pressure along the wellbore. This "overbalanced" pressure strengthens the wellbore (helping to avert wall cavings) and prevents a formation fluid influx or "kick" into the wellbore. However, the overbalanced pressure also strengthens the formation face being drilled similar to the strengthening of the walls of the drilled well. This now "harder" drilling face drills at a lower rate of penetration, increasing drilling time and cost.
Reducing the normally overbalanced pressure to minimize rotary drilling cost increases the risk of wellbore caving damage and well control problems. Thus, a drilling operator has to consider the conflicting fluid pressure needs of maintaining the integrity of the bore and economically drilling the formation face.